Heating elements normally have to be joined to electrically conducting bodies of sizes that are typically larger than the elements themselves called terminals. Terminals are provided so that while the heating element is being heated by passage of current, the terminal remains cool because its larger cross section enables the terminal to carry a lower current density. This in turn allows for the safe and efficient attachment of leads from the power source to such a terminal; in the absence of which terminals the leads would have to be directly attached to the heating element.
Typical processes of attachment of terminals to ceramic and metal-ceramic heating elements are known to be costly. This high cost enhances the total cost of the use of ceramic and metal-ceramic heating elements in radiant heaters and such other applications. Thus there is a need for novel and less expensive processes to accomplish the task of joining ceramic and metal-ceramic heating elements to terminals.
This problem of attaching terminals to ceramic and metal-ceramic heating elements is known to be especially challenging with respect to heating elements made of molybdenum disilicide (MoSi.sub.2), silicon carbide (SiC) and composites thereof. This is because MoSi.sub.2 has a high melting point (2020.degree. C.) and because at temperatures above 800.degree. C., in an oxidizing atmosphere, the surface of a MoSi.sub.2 element gets covered with SiO.sub.2. Hence joining techniques for such elements require high temperatures, normally above 1600.degree. C., and protective atmospheres.
MoSi.sub.2 has the following properties that make it an ideal metal-ceramic for use in applications such as top glass cooking stoves, which utilize a radiant plate placed under a glass ceramic transparent top with the heating element comprising an electrically heated body, supported by an insulating base: (1) The resistivity of MoSi.sub.2 increases with temperature. (2) The resistivity-temperature curve for a MoSi.sub.2 heating element is very steep, with the resistivity ratio at 20.degree. C. to 1500.degree. C., being about IO. (3) The long time working temperature of MoSi.sub.2 elements is well above 1350.degree. C. Hence when a MoSi.sub.2 heating element is connected to a constant voltage source, the power required (?) will initially be high at low temperatures. As heating progresses, the power (current.times.voltage) required decreases as the radiant body temperature increases. The above described resistance-temperature characteristics thus enable a MoSi.sub.2 heating element to be heated to above 1350.degree. C. immediately when the power is turned on.
A typical top glass cooking stove assembly when activated emits energy through the glass ceramic thereby heating the bottom surface of a utensil placed directly thereupon. Normal metallic heating elements are not suitable for such an application because of the low surface temperature which is possible to be generated upon the heating of a metallic heating element and also the related slow response to further heating. A much faster response can be obtained by (1) increasing the surface temperature of the radiant heating elements to maximize the radiated energy, and (2) minimizing the thermal mass of the heating bodies in order to reduce the thermal inertia of the system. For the reasons mentioned above, MoSi.sub.2 heating elements are being actively considered for such applications. It is to be understood that this possible use of MoSi.sub.2 heating elements is provided for purposes of illustration only, and should not be construed to be limiting.
Conventional techniques for joining molybdenum disilicide heating elements to electrical terminals utilize complex and expensive techniques such as electron beams, laser or plasma welding technology. An example is provided in U.S. Pat. No. 3,668,599 ('599) of Jun. 6, 1972, issued to Niles Gustav Schrewelius. The '599 patent discloses a device which comprises an array of parallel MoSi.sub.2 heating resistance element rods coupled in series by connecting adjacent rod ends together in pairs at one end of the array and, at the other end connecting adjacent rods ends together in pairs, staggered with respect to those at the first end. The connection is made by means of a flame-sprayed layer of MoSiAl or MoSi.sub.2. The '599 patent utilizes a very high cost, high energy beam machine like a thermal spray gun. The use of a thermal spray gun is a very slow process. Implementing the '599 patent in a typical manufacturing shop would involve expenditure of considerable time and effort and would also be quite expensive. Similarly the use of an electron beam or laser woud be very expensive.
The novel micropyretic synthesis based techniques and unique compositions disclosed in the present invention will eliminate the above problems associated with prior art techniques for joining MoSi.sub.2 radiant heater elements to electrical terminals.
Another problem associated with cooking stove type applications is that the wire diameter used in such applications is typically only about 0.7 to 1 millimeter. The conventional joining techniques, such as the ones described above in the '599 patent, are difficult to implement for such small diameters on account of the brittleness of MoSi.sub.2. The present invention will also alleviate this problem by providing the advantage of easy handling, air atmosphere firing (the present invention does not require a special atmosphere during the joining step), instant joining onto the required geometry and shape.
Co-pending application U.S. Ser. No. 07/847,782 ('782) provides a novel technique to make electrical heating elements which may be used up to 1900.degree. C. New methods are also provided for manufacturing ceramic composites, which may be used as both electrical heating elements and oxidation resistant materials. There are also provided in the '782 application, compositions for manufacturing the above mentioned ceramic composites and heating elements. The '782 application is incorporated by reference into the present application.
While the '782 application provides methods for the manufacture of electrical heating elements and the like, there is no suggestion that this method i.e. micropyretic synthesis would be applicable in joining heating elements produced in accordance with the '782 application to electrical terminals. Also there is no suggestion that electrical terminals can be manufactured using ceramic and metal ceramic compositions.
The present invention also provides a heating element assembly for a radiant heating device, which assembly utilizes the method and compositions of this invention. In general this assembly includes a pair of terminals for connecting the heating element to the source, a plurality of heating elements and a plurality of coolers. The coolers are used to connect the heating elements to each other. The coolers act like intermediate terminals. These intermediate terminals may also be used to connect the heating elements to the surface below so as to provide mechanical support, as they remain cooler than the heating elements themselves.
Thus, the present invention provides a low-cost, novel technique for joining ceramic and metal ceramic heating elements to electrical terminals, which eliminates or alleviates several of the problems associated with prior art techniques. The present invention also provides optimal ceramic and metal ceramic compositions for manufacturing electrical terminals which compositions will be especially suited for effecting the micropyretic synthesis based joining techniques described herein. Finally the present invention provides a heating element assembly for a radiant heating device, which assembly utilizes the method and compositions of this invention.